The present invention relates to a retractable anchoring device adapted to rigidly position by intermittance an instrument inside a cavity by application of movable arms against the wall thereof.
More particularly, the invention concerns an anchoring device with retractable arms, adapted to maintain intermittently inside a drilling well, wherein it is lowered at the end of a cable, a sonde equipped with a measuring apparatus particularly for use in seismic prospecting.
Such a sonde may be equipped, for example, with seismic sensors (geophones or hydrophones) for receiving seismic waves of the longitudinal type (P waves), or of the transverse type (S waves), transmitted from a source of seismic shocks placed at the surface of the earth close to the well. The sonde sensors sense the waves directly transmitted from the surface and the echoes of the transmitted waves on the underground reflecting layers. The direct waves are used for determining the sound propagation velocity through the traversed formations, as well for the P waves as for the S waves, (i.e., acoustic coring). The echoes received by the sonde sensors make it possible to locate the actual position of the reflecting layers and, optionally, to estimate the sound propagation velocity in certain zones located below the bore hole.
The sonde may also be equipped with acoustic waves transmitters and receivers for determining certain characteristics of the formations traversed by the borehole at the vicinity thereof, (i.e. acoustic logging).
The systems for anchoring these devices, lowered into holes or wells drilled through the earth at the end of a cable, generally comprise one or more pivotable arms which can be moved aside or retracted at will by driving means. These means may be merely mechanical and comprise, for example, and endless screw driven in rotation by an electric motor controlled from the surface. In this case, the rotation of the screw displaces a nut which is integral with one or more links connected to the pivoting arms. For purposes of security, it must always be possible to retract the arms, even in the case of stopping of the driving motor and, accordingly, it is generally preferred to rigidly secure the ends of the links to a ring on which a spring bears. The expansion of the spring has the effect of pushing the ring and moving the arms aside. The motor is only used to retract the arms. The nut motion, controlled by the rotation of the endless screw, results in the ring being driven off and in the compression of the spring. In the case where the motor is stopped, the back motion of the ring due to the compression of the spring provides for the retraction of the arms. By making use of such an arrangement, the bearing force of the arms against the walls of the well is the lower as they are more spaced aside and the spring becomes more expanded, and it in some cases may even become insufficient to ensure a satisfactory anchoring of the sonde system on the walls of the well.
In other prior systems, the driving means for actuating the arms in rotation are of the hydraulic type. An electric motor drives a hydraulic pump feeding, with fluid under pressure, either a single jack replacing the assembly of the endless screw and the nut and acting on the links, or jacks directly acting on the pivoting arms. The hydraulically controlled systems are advantageous since the force applied to the arms is independent from their spacing with respect to the sonde body, but they are difficult to operate inasmuch as they make use of hydraulic pumps whose operation becomes critical in the severe pressure and temperature conditions prevailing at the bottom of a well. As a matter of fact, it is known that, in a bore hole, at a 3000 m depth for example, with a mud of density 1.3, the static pressure may reach 40 MPa and the temperature may rise up to 150.degree. or 200.degree. C. so that it is difficult under these conditions to maintain a sufficient viscosity of the liquid of the pump circuit.